What is the difference between

Question

I saw these two parameters in a C example in a C book but the author didn\'t elaborate what the difference between the two are. I know that %f specifies that a

Answer 1

For output using the printf family of functions, the %f and %lf specifiers mean the same thing; the l is ignored. Both require a corresponding argument of type double — but an argument of type float is promoted to double, which is why there’s no separate specifier for type float. (This promotion applies only to variadic functions like printf and to functions declared without a prototype, not to function calls in general.) For type long double, the correct format specifier is %Lf.

For input using the scanf family of functions, the floating-point format specifiers are %f, %lf, and %Lf. These require pointers to objects of type float, double, and long double, respectively. (There’s no float-to-double promotion because the arguments are pointers. A float value can be promoted to double, but a float* pointer can’t be promoted to a double* because the pointer has to point to an actual float object.)

But be careful using the scanf functions with numeric input. There is no defined overflow checking, and if the input is outside the range of the type your program’s behavior is undefined. For safety, read input into a string and then use something like strtod to convert it to a numeric value. (See the documentation to find out how to detect errors.)